Honyaki knives

Knife

A knife is a cutting tool with an exposed cutting edge or blade, hand-held or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools...

 are forged from one single material, usually high-carbon steel. The finest honyaki (mizu-honyaki) are then differentially-hardened

Differential hardening

Differential hardening is a method used in forging swords and knives to increase the hardness of the edge without making the whole blade brittle. To achieve this, the edge is cooled more rapidly than the spine by adding a heat insulator to the spine before quenching. Clay or another material is...

, the same method used for traditional katana

Katana

A Japanese sword, or , is one of the traditional bladed weapons of Japan. There are several types of Japanese swords, according to size, field of application and method of manufacture.-Description:...

. Their sharpness is the longest lasting of all Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

ese blades. They are extremely difficult to forge

Forge

A forge is a hearth used for forging. The term "forge" can also refer to the workplace of a smith or a blacksmith, although the term smithy is then more commonly used.The basic smithy contains a forge, also known as a hearth, for heating metals...

, requiring a high level of skill and experience. They are also very difficult to sharpen and maintain, and easily damaged if not properly used. They are also more expensive than other knives (costing over $1000 for a 240mm gyuto), such as Kasumi knives, which are made out of two materials and are easier to forge and maintain.